Plant for Heating by Combustion of a Solid Fuel

ABSTRACT

The invention relates to a heating plant. The plant comprises a combustion chamber for combustion of a solid fuel, an outlet from the combustion chamber, and a heat exchanger containing a cooling medium and arranged in connection with the combustion chamber. A flue duct is connected to the outlet of the combustion chamber and passes through the heat exchanger. Downstream the heat exchanger, a cyclone is arranged to separate ash from the flue gases, and a flue fan is arranged to draw flue gases through the flue duct. The flue duct and the flue fan are dimensioned to maintain a high velocity of flow in the flue duct, all the way from the outlet of the combustion chamber and up to the cyclone, in order to achieve a blasting effect on the inside of the flue duct by ash particles being drawn through the flue duct.

TECHNICAL FIELD

The present invention relates to a plant for heating by combustion ofsolid fuel, in which flue gases from a combustion chamber are led froman outlet of the combustion chamber and through a heat exchanger inwhich heat from the flue gases is transferred to a cooling medium.

BACKGROUND OF THE INVENTION

Fuel fired boilers are used to heat houses e.g., heat being generated bycombustion of solid fuels such as firewood or fuel pellets. By fuelpellets is understood pieces of combustible material achieved bypressing comminuted material such as sawdust at high pressure, to formcontinuous pieces that can be used as a fuel. In a fuel fired boiler,fuel is combusted to generate heat energy. In turn, the heat energy canbe used to heat water or some other liquid medium that can be circulatedto give heat to different parts of a building. The fuel combustion takesplace in a combustion chamber and during combustion of the fuel, fluegases are formed and led away from the combustion chamber. As the fluegases contain a considerable amount of heat energy, it is desirable tobe able to use this heat energy. Therefore, it is customary to lead theflue gases through flue ducts/flues in the form of a conduit that passesa heat exchanger. The heat exchanger can be seen as a cooling plant forthe hot flue gases, in which the heat content of the flue gases istransferred to a cooling medium such as water that accordingly is beingheated by the hot flue gases. Thereafter, the thus heated cooling mediumcan be circulated to heat different parts of a building, where after thecooling medium is returned to the heat exchanger. In order to transferas much as possible of the flue gas heat energy to the cooling medium,the flue duct should be of good thermal conductivity.

Ashes are formed when fuel pellets are combusted in the combustionchamber. Generally, one distinguishes ashes in the combustion chamberfrom ashes going out with the flue gases. The ashes that remain in thecombustion chamber are usually called bottom ash, and the ashes that goout with the flue gases are called fly ash. Accordingly, ash particlesin the fly ash will go out with the flue gases in the flue duct. The flyash going out with the flue gases tends partly to stay in the flue ductor the flues to settle on the inside thereof. Coatings on the inside ofthe flue ducts can also be formed by soot. Soot is a coating caused byimperfect combustion. Coatings of fly ash and/or soot on the inside ofthe flue duct act isolating and impair the heat transfer from the fluegases to the cooling medium. When heat exchange is impaired, thetemperature of the output flue gases rises, since more of the heatcontent stays in the flue gases instead of being transferred to thecooling medium. Accordingly, the efficiency of the plant is impaired.The ashes formed at combustion also give problems in that the bottom ashmust be removed from the combustion chamber.

Hence, it is an object of the present invention to achieve a heatingplant using solid fuel to heat a cooling medium, in which the heattransfer to the cooling medium has been rendered more effective. Yetanother object of the invention is to provide an efficient way ofremoving ashes from the combustion chamber. It is also an object of thepresent invention to prevent fly ash and/or soot from forming coatingson the inside of the flue duct. These and other objectives are achievedby the present invention, as will be described in the following.

ACCOUNT OF THE INVENTION

The invention relates to a heating plant comprising a combustion chamberfor combustion of solid fuel, in which combustion chamber the solid fuelis combusted. An outlet of the combustion chamber is suitably arrangedat the bottom of the combustion chamber. The plant also comprises a heatexchanger in the form of a cooling plant containing a cooling medium,i.e. the cooling plant is designed to hold a cooling medium. A flue ductis connected to the outlet of the combustion chamber, which flue duct isarranged to pass through the cooling plant, such that heat energy in theflue gases from the combustion chamber can be transferred to the coolingmedium. Downstream the cooling plant, there is a separation device, e.g.a cyclone, for separation of ashes from the flue gases. A flue fan isarranged to draw flue gases through the flue duct. The fan can bearranged in connection with the separation device (a cyclone e.g.).Accordingly, the fan is arranged in connection with the flue duct,downstream the heat exchanger. An ash collector is preferably providedin connection with the separation device. According to the invention,the flue duct and the fan are dimensioned to maintain a velocity of flowof at least 15 m/s in the flue duct, all the way from the outlet of thecombustion chamber to the ash separation device, or at least to thepoint where the flue duct exits the heat exchanger. Preferably, the flueduct and the fan are dimensioned to be able to maintain a velocity offlow of at least 18 m/s in the flue duct, all the way from the outlet ofthe combustion chamber and to the separation device, or to the fan, if adedicated separation device is lacking.

Suitably, the ash collector is detachably arranged at a lower end of aconduit from the separation device.

Advantageously, the device according to the invention is designed suchthat parts of the flue duct project from the cooling plant and can bedetached from the rest of the flue duct. Thereby, the flue duct will beaccessible for cleaning.

According to a particularly advantageous aspect, the plant onlycomprises a single flue duct, which means that all flue gases will passthe same duct.

According to another advantageous aspect of the invention, a movable ashscraper can be provided in the combustion chamber in order to shoveashes in a direction towards the outlet of the combustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of an example of a prior art combustiondevice.

FIG. 2 shows the same plant as in FIG. 1, partly in cross-section and ina front view.

FIG. 3 shows a cross-section of an embodiment of the present invention.

FIG. 4 shows a part of the plant shown in FIG. 3, in a perspective view.

FIG. 5 shows a second embodiment of the invention.

FIG. 6 shows a detail of an advantageous embodiment of the invention.

FIG. 7 shows an advantageous aspect of the embodiment shown in FIG. 6.

FIG. 8 shows a fan curve for the flue fan of the plant.

DETAILED DESCRIPTION OF THE INVENTION

First, a plant according to prior art is described with reference toFIGS. 1 and 2. The plant 1 shown in FIG. 1 comprises a combustionchamber 2 in which fuel pellets are combusted, usually by use of aburner (not shown). Flue gases from the combustion leave the combustionchamber via an outlet 3, to pass a flue duct 6. As indicated in FIG. 1,the flue duct 6 branches in a plurality of ducts 6. The flue ducts 6 arepassing a heat exchanger 5 in which the heat content of the flue gasesis transferred to a cooling medium, usually water, in the heat exchanger5. As explained above, fly ash and soot in the flue gases tend to stayin the flue duct/flue ducts 6, settling on the inside thereof. Inexisting plants, the velocity of flow of the flue gases is comparativelylow, in practice about 6 m/s. The inventor of the present invention hasfound that if the velocity of flow in the flue duct 6 is increased,essentially all ash can be removed through the flue duct 6, such thatalso the bottom ash is removed through the flue duct 6. The inventor hasfound that the bottom ash can act then to blast the inside of the flueduct 6, such that the bottom ash prevents formation of coatings of flyash on the inside of the flue duct 6. As such, a high velocity of flowwill also diminish the risk of fly ash remaining as a coating on theinside of the flue duct 6. Hence, at adequately high velocity of flow,the problem of coatings in the flue duct can be eliminated orconsiderably decreased, such that only a very small amount of the flyash remains in the flue duct 6 in the form of coatings on the insidewalls of the flue duct 6.

Therefore, the present inventor has found that the velocity of flow inthe flue duct should be increased to at least 15 m/s. By practicalexperiments, the inventor has found that particularly good results canbe achieved when the velocity of flow in the flue duct 6 is increased to18 m/s or more.

The invention will now be explained in greater detail with reference toFIG. 3. The heating plant 1 according to the invention comprises acombustion chamber 2 for combustion of solid fuel, in particular fuelpellets of sawdust and similar materials, in which combustion chamberthe solid fuel is combusted when the plant is used. The figure showsschematically that a burner 17 is arranged in the combustion chamber(the boiler), and that the burner 17 can be fed with fuel pellets by aconveyor 18, such as a worm conveyor. An outlet 3 from the combustionchamber 2 is arranged to enable removal of hot flue gases from thecombustion chamber 2. In preferred embodiments, the outlet 3 is arrangedat the bottom 4 of the combustion chamber 2 shown in FIG. 3. Inconnection with the combustion chamber 2, a heat exchanger 5 containinga cooling medium is arranged. It is to be understood that duringoperation of the plant, the cooling medium circulates in a closed cycle.Advantageously, the cooling medium is water. A flue duct 6 is connectedto the outlet 3 of the combustion chamber 2, which flue duct 6 passesthrough the cooling plant 5, such that heat energy in the flue gasesfrom the combustion chamber 2 can be transferred to the cooling medium.In many realistic applications, the temperature of the flue gasesleaving the combustion chamber 2 can be about 500° C. at the outlet 3,while effort is made to cool the flue gases in the heatexchanger/cooling plant 5 such that the temperature of the flue gasesare considerably lower after the heat exchanger/cooling plant 5, forexample about 110° C. Downstream the cooling plant 5, a device 7 isarranged to separate ashes from the flue gases. Such a device forseparation of ash from the flue gases can be a cyclone 7. A cycloneseparates ashes from the flue gases by use of centrifugal forces, in amanner known per se, and therefore no further explanation is needed inthis connection. Ashes separated from the flue gases in the separationdevice 7, will sink down in a vertical conduit 12. A fan 8 is arrangedin connection with the ash separator 7, to draw flue gases via the flueduct 6. It should be understood that then, the fan 8 is arranged inconnection with the flue duct 6 at a location downstream the heatexchanger 5. An ash collector 9 is arranged in connection with the ashseparator 7, which ash collector 9 is preferably detachably arranged ata lower end 11 of the conduit 12 from the ash separator 7.

The flue duct 6 and the flue fan 8 (suitably including a not shown drivefor the fan) are dimensioned to maintain a velocity of flow of at least15 m/s in the flue duct 6, all the way from the outlet 3 of thecombustion chamber 2 and all the way to the ash separator 7, or to thefan 8, if the plant lacks an ash separator 7. Preferably, the flue duct6 and the fan 8 are dimensioned to be able to maintain a velocity offlow of at least 18 m/s in the flue duct 6, all the way from the outlet3 of the combustion chamber 2 and all the way to the fan 8 or to the ashseparator 7, if the plant has one. In experiments, the inventor hasfound that at a velocity of flow of 18 m/s, there was considerably lessash coatings on the inside of the flue duct 6 as compared to at avelocity of flow of 6 m/s.

The velocity of flow in the flue duct 6 depends both on the flue ductitself and on the fan 8. It is now referred to FIG. 8, showing fancurves at different rotational speed. The volume flow Q maintained bythe fan 8 depends on the pressure drop P against which the fan operates,as well as, for a given fan, its rotational speed. The pressure dropfrom the outlet 3 of the combustion chamber 2 and up to the fan 8,depends among other things on the cross-sectional area and length of theflue duct 6, as well as on the friction in the flue duct. A more ruggedsurface on the inside of the flue duct results in an increased pressuredrop. In the same way, a longer duct 6 or a duct 6 of smallercross-sectional area, results in a larger pressure drop to be overcomeby the fan 8. As already mentioned, the volume flow also depends on therotational speed at which the fan 8 operates. FIG. 8 shows three curvesV₁, V₂ and V₃, which different curves represent the performance of oneand the same fan at different rotational speeds. It is realised thatcurve V₁ shows the conditions at a relatively low rotational speed,while curve V₂ shows the conditions at a higher rotational speed. CurveV₃ represents an even higher rotational speed. The velocity of flow canbe calculated as the volume flow divided with the cross-sectional areaof the flue duct 6. The exact dimensions of the flue duct 6 may ofcourse vary within wide limits. In a plant intended to heat asingle-family house, the flue duct may e.g. be of a diameter in themagnitude of 50 mm. The fan 8 in FIG. 3, shown to be positioned inconnection with the ash separator 7, can be controlled by a controlmember 15 that may be a PC for example. FIG. 3 shows schematically thata pressure gauge 16 is arranged in or in connection with the combustionchamber 2. Via a connection 50, the pressure gauge 16 is connected withthe control member 15. In a preferred embodiment, the control member 15can be set for a desired vacuum value in the combustion chamber 2. Ifthe vacuum in the combustion chamber 2 is less than the desired value,the control member 15 will increase the rotational speed of the fan 8until the vacuum in the combustion chamber 2 returns to the desiredvalue. Thereby, flue gases are avoided from “smoking out”, i.e. fromcoming out at the burner 17 instead of through the flue duct 6.

As shown in FIG. 3, a movable ash scraper 13 is suitably provided in thecombustion chamber in order to shove bottom ash B in a direction towardsthe outlet 3 of the combustion chamber 2. The ash scraper 13 is arrangedto move forwards and backwards in the direction of the arrow A in FIG.3. Suitably, the scraper 13 has a width corresponding to the inner widthof the combustion chamber 2. Then, the bottom ash that is not directlysucked to the outlet 3 by the draught of the fan 8, can be shoved by thescraper 13 to the outlet 3, from where it can be led on through the flueduct 6.

The maintaining of an appropriate and predetermined vacuum in thecombustion chamber can also ensure that the velocity of flow in the flueduct 6 is maintained at an appropriate level. It should be understoodthat the controlling of the fan 8 can be achieved also by control of theblade angle, as is known per se. Then, the control member 15 willcontrol the blade angle in response to a signal from the gauge 16. As analternative, or as a supplement to the vacuum measuring in thecombustion chamber 2, it may be conceived to measure the velocity offlow in the flue duct 6, such that the control member controls the fandepending on the rate measured.

The fan 8 can also be controlled by on/off-controlling.On/off-controlling can be particularly suitable for house boilers.

It should also be understood that in many applications, such as forhouse boilers, the fan 8 need not necessarily operate continuously at arotational speed high enough for bottom ash to be drawn through the flueduct all the time. It is conceivable then that in connection with thestarting up of the fan 8, it is allowed to operate at maximum rotationalspeed, such that bottom ash is sucked through the flue duct 6, to exerta blasting effect on the inside of the flue duct 6. After some time,such as 1-2 minutes, the rotational speed can be lowered in order todecrease the energy consumption. Accordingly, theself-sweeping/self-cleaning function of the plant according to theinvention need not necessarily be used continuously during operation. Ifa movable ash scraper 13 is used, it is also conceivable that themovable ash scraper 13 only moves in connection with the starting up ofthe plant, in order to shove bottom ash towards the outlet 3 inconnection with the starting up. Thereafter, the ash scraper 13 canreturn to its starting position and stay there until the next startingup. In larger plants operated continuously for a longer time, the ashscraper 13 can move back and forth at predetermined time intervals.

According to the prior art represented in FIGS. 1 and 2, the flue gasesare led through a plurality of parallel ducts 6, which means that notall flue gases will pass the same way through the heat exchanger 5. Ithas been shown that this can lead to some unfavourable results. This isthe case since the inventor of the present invention has found that theflue gases tend to distribute unequally over the different ducts, suchthat the flow will be lower in some ducts 6. If there are severalparallel conduits, the gases can, due to turbulence, choose the easiestway, whereas the more sluggish way tends after some time to beincreasingly blocked up by fly ash.

Therefore, in a preferred embodiment, the inventor has considered analternative design that will be explained in more detail in thefollowing. According to such a particularly preferred embodiment of theinvention, the device comprises only a single flue duct 6. By using onlya single flue duct 6, through which all flue gases have to pass, theadvantage is attained that the flow as well as the velocity of flow canbe more effectively controlled than in case of several parallel flows.

It should also be understood that the cross-sectional area of the flueduct need not necessarily be constant all the way from the outlet 3 ofthe combustion chamber. When the flue gases pass through the flue duct6, their temperature will decrease considerably. When temperature sinks,the volume of the flue gases decreases. In order to maintain thevelocity of flow in the flue duct 6, it may therefore be suitable to letthe cross-sectional area of the flue duct to decrease in the directionof flow of the flue gases. If the cross-section of the flue duct iscircular, its diameter can hence be larger immediately downstream theoutlet 3 of the combustion chamber than at the connection to the ashseparator 7. It is realised that consideration thereto will have to betaken when dimensioning the flue duct as well as the fan and its powerrequirement, in order to ensure that the required velocity of flow canbe achieved.

As is shown in FIG. 6, parts 10 a, 10 b, 10 c of the flue duct 6 can, inthe preferred embodiment of the invention, project from the heatexchanger/cooling plant 5 and be arranged to be detachable from the restof the flue duct 6, such that the flue duct 6 is accessible forcleaning. The parts 10 a, 10 b, 10 c can be detachable pipe bends 10 a,10 b, 10 c. If there is a need of cleaning the flue duct 6, the pipebends 10 a, 10 b, 10 c can be easily removed from the rest of the flueduct 6. Then, it is easy to insert a brush 13 in the flue duct 6, as isshown in FIG. 7, where after the brush 13 (or some other tool) is usedto remove coatings from the inside of the flue duct 6. It should beunderstood that by the increased velocity of flow in the flue duct, theneed of such cleaning is considerably reduced, but that the detachablepipe bends 10 a, 10 b, 10 c enable easy cleaning if the need arises allthe same.

It should be understood that embodiments are conceivable in which theflue duct 6 does not project from the heat exchanger 5 until it exitstherefrom. Embodiments are conceivable for example in which the heatexchanger/cooling plant 5 completely surrounds the combustion chamber 2,and in which the flue duct 6 forms a spiral around the combustionchamber 2. Then, the flue duct would follow a continuous curve.

FIG. 3 shows that the cyclone 7 has a vertical conduit 12, the lower end11 of which being connected to an ash collector 9. Preferably, thecollector 9 is detachably arranged at the lower end 11. By the ashcollector 9 being separate from the combustion chamber 2, the advantageis attained that ash emptying can take place without direct access ofthe combustion chamber 2 itself, and without stopping the combustion inthe combustion chamber 2. The ash collector 9 is a preferred component,but embodiments are conceivable without a dedicated ash collector.

An alternative embodiment of the invention will now be explained withreference to FIG. 5. In the embodiment shown in FIG. 3, the combustionchamber 2 is positioned vertically below the heat exchanger 5 and theflue gases are drawn vertically up from the outlet 3 of the combustionchamber. In the embodiment according to FIG. 6, the combustion chamber 2is however positioned vertically above the heat exchanger 5, andaccordingly the flue gases will be sucked vertically down through theflue duct 6. Thereby, the advantage is attained among other things, thatit is easier to draw the bottom ash.

An additional aspect of the invention will now be explained withreference to FIGS. 4 and 5. In FIG. 3, it is shown how the pipe bends 10a, 10 b, 10 c of the flue duct 6 are all bent in a vertical plane. InFIGS. 4 and 5 it is suggested that parts of the flue duct 6 can bepositioned in the same horizontal plane, connected to each other by pipebends 10 bent in a horizontal plane or an at least partially horizontalplane. Then, the flue duct 6 can have parallel parts in which the fluegases flow in opposite directions. Thereby, heat transfer is increased.

Embodiments of the invention are conceivable in which no cyclone isused. One example of such a conceivable embodiment comprises the optionthat flue gases and ashes are discharged directly into the environment.In such an embodiment, the velocity of flow in the flue duct need onlybe maintained up to the point at which the flue duct leaves the heatexchanger, or alternatively up to the fan 8.

A number of advantages are obtained by the invention. By operation athigher velocity of flows, heat transfer is increased which means thatthe heat transfer area can be made smaller, i.e. the flue duct can bemade shorter. The bottom ash forms a blasting medium that sweeps awaysoot residues in the flue duct. Accordingly, by the plant beingself-sweeping, a low and constantly temperature can be achieved in theoutput flue gases, and consequently a better efficiency in the plant. Asingle fan can be used. The risk of “smoking”, such that the flue gasesexit at the burner instead of via the flue duct, is eliminated orconsiderably reduced.

It should be understood that dimensions and reachable efficiency levelscan vary within very wide limits.

It should be understood that in preferred embodiments, the heatexchanger/cooling plant 5 is arranged in direct connection with thecombustion chamber 2. Embodiments are conceivable however in which theheat exchanger is arranged at a distance from the combustion chamber,such that the flue duct 6 will pass through air or ground e.g., beforeit reaches the heat exchanger.

If the outlet 3 of the combustion chamber 2 is arranged at the bottom ofthe combustion chamber, the advantage is attained that it is easier todraw bottom ash into the outlet, to be led through the flue duct 6.

It should be realised that the idea of an ash scraper arranged to shovebottom ash to an outlet in the bottom of the combustion chamber can beused independent of the other design of the plant, for example toprepare for a later conversion of a plant. If an ash scraper is arrangedto shove ashes to the outlet, this can contribute to evacuation of atleast a part of the bottom ash also in plants in which the velocity offlow is too low for any true blasting effect to be achieved.

It should also be realised that the idea of controlling the rotationalspeed of the fan and/or its blade angle, as a function of a vacuummeasured in the combustion chamber or a velocity of flow measured in theflue duct, can be applied independently of the other design anddimensioning of the plant.

The invention can also be defined in terms of a method ofinstalling/mounting a heating plant, whereby in connection with theinstalling measures are undertaken that are necessary for the plant tooperate as described above, i.e. by choice of pipe length, dimensionsand fan in order to ensure the above described operation.

The invention can also be defined in terms of a method of operating aheating plant, whereby the plant is operated as described above.

It should also be realised that the plant according to the invention cancomprise software, e.g. in the control member 15. The software can beused to control the fan depending on the vacuum measured in thecombustion chamber and/or the velocity of flow in the flue duct.

It should be understood that in the dimensioning, it is suitable to takeinto consideration the temperature of the flue gases exiting thecombustion chamber in each single operating case. Depending on a numberof factors, the temperature of these can vary, and in principle it canbe as low as 100° C., but more realistically it is 400-500° C. or more.

1-10. (canceled)
 11. A heating plant comprising a combustion chamber forcombustion of a solid fuel, an outlet from the combustion chamber, aheat exchanger arranged in connection with the combustion chamber, aflue duct connected to the outlet of the combustion chamber, which flueduct passes through the heat exchanger, such that heat energy in fluegases from the combustion chamber can be transferred to a cooling mediumin the heat exchanger, an ash separator for separation of ash from theflue gases and positioned downstream from the heat exchanger, and a fanarranged in connection with the ash separator in order to draw the fluegases through the flue duct, wherein the flue duct and the fan aredimensioned to maintain a velocity of flow of at least 15 m/s in theflue duct, all the way from the outlet of the combustion chamber and upto the ash separator, in order to achieve a blasting effect on theinside of the flue duct by ash particles being drawn through the flueduct.
 12. A heating plant according to claim 11, wherein the flue ductand the fan are dimensioned to maintain a velocity of flow of at least18 m/s all the way from the outlet of the combustion chamber and up tothe ash separator.
 13. A heating plant according to claim 11, whereinparts of the flue duct project from the heat exchanger, which parts aredetachable from the rest of the flue duct, such that the flue ductbecomes accessible for cleaning.
 14. A heating plant according to claim11, wherein the heating plant comprises no more than a single flue duct.15. A heating plant according to claim 11, wherein the ash separator isa cyclone, the heating plant further comprising an ash collector that isdetachably arranged at a lower end of a conduit from the cyclone.
 16. Aheating plant according to claim 11, wherein a movable ash scraper isarranged in the combustion chamber in order to be able to shove ashestowards the outlet of the combustion chamber.
 17. A heating plantaccording to claim 11, wherein the outlet from the combustion chamber ispositioned at a bottom of the combustion chamber.
 18. A heating plantaccording to claim 11, wherein the fan is arranged to be operated with avariable rotational speed.
 19. A heating plant according to claim 11,wherein the fan is arranged to be operated with a variable blade angle.20. A heating plant according to claim 18, wherein the fan is arrangedto be controlled as a function of a pressure level in the combustionchamber.
 21. A heating plant according to claim 19, wherein the fan isarranged to be controlled as a function of a pressure level in thecombustion chamber.